Electronic apparatuses and methods for providing standby signals are utilized in many types of present day communication equipments. Such apparatuses include circuits and systems which enable the conservation of electrical power, which may be supplied by a battery, when either a too weak signal or no signal is being received by a receiver associated with the communication equipment. Moreover, such apparatuses prevent false triggering of noise sensitive circuits by rendering them inoperative as the signal-to-noise ratio reaches a critically low magnitude. Noise sensitive circuits may include digital logic systems, for instance, which could generate improper command signals in response to noise.
Prior art standby signal generating systems and methods have been utilized with phase modulation (PM) receivers which include a superheterodyne type frequency converter coupled to an intermediate frequency (IF) amplifier. Such standby systems include an amplitude detector connected from the output terminal of the IF back to the gain control terminal of an amplifier which is situated in the signal path before the output terminal of the IF amplifier. The amplitude detector develops an automatic gain control (AGC) signal in response to the amplitude of the carrier component present in the signal at the output terminal of the IF amplifier. More particularly, as the carrier signal strength tends to decrease, the magnitude of the AGC voltage changes to cause the increase in gain necessary to maintain the carrier signal at the output of the IF amplifier at a substantially constant magnitude. The AGC voltage versus signal strength characteristic for such receivers is substantially linear and flat. A magnitude sensitive trigger circuit is generally coupled to the detector and produces the standby signal in response to the magnitude of the AGC signal crossing a predetermined threshold. The standby signal causes the shutdown of selected portions of the signal processing equipment connected to the receiver, for the purposes set forth above.
The above described prior art standby system and method has disadvantages which makes them unsuitable for use in some high performance equipment such as telemetry transponders being utilized in satellites. More specifically, the magnitude of the AGC signal in the aforementioned prior art system varies with change in gain and other parameters of any of the receiver stages. Since this gain is a function of temperature and age, the shutdown threshold level for the trigger circuits in prior art systems are usually chosen quite conservatively which results in the sacrifice of operating performance particularly at low signal strengths.